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The former Deputy Prime Minister will earn more than £1million a year in the role, which he accepted after "months of wooing" by Mark Zuckerberg, Facebook's chief executive.

Sir Nick said he will give up his role in campaigning for a second referendum with immediate effect as he admitted that there is a limit to how much people from the "political past" like him can influence the Brexit debate.

He will be paid to lobby in Europe and the US on behalf of Facebook, where the company is facing investigations over its tax affairs and data breaches.

He was previously a prominent critic of Facebook, which paid only £7.4million in tax in the UK last year despite recording £1.3billion in sales.

"I'm not especially bedazzled by Facebook. I actually find the messianic Californian new-worldy-touchy-feely culture of Facebook a little grating," he said two years ago.

"Nor am I sure that companies such as Facebook really pay all the tax they could — though that's as much the fault of governments who still haven't got their tax act together.'

“The jiggery-pokery of tax planning by the large multinational tech companies is also guaranteed to inflame public opinion. It may all be impeccably legal but the impression that less tax is being paid than can reasonably be expected remains."

His appointment surprised senior Liberal Democrats. Vince Cable, the party's leader, said he will write to Sir Nick "urging him to make sure Facebook co-operates with attempts to make sure they pay their fair share of tax".

Nigel Evans, a Tory MP, said: "This is deeply hypocritical. He has been critical about companies that try and reduce their tax and he is now running to the door of Mark Zuckerberg.

"Quite frankly we all know he has the ability to say one thing and do another. He's now their champion. Pass the sick bag."

1/ Bit more detail on @nick_clegg's move to @facebook. My @BBCNews blog up shortly. Before that, here's a digest, based on a long-ish conversation with him a short while ago...

The former Liberal Democrat MP, who lost his Sheffield Hallam seat to Labour last year will start working for Facebook on Monday and move with his family to Palo Alto in California in the New Year.

His wife, Miriam Gonzalez, will leave her current role as a lawyer at Dechert, although she will continue with her charitable work. The couple have three children aged 16, 14 and nine.

His eldest son, Antonio, is in remission after a battle with Hodgkin Lymphoma and Mr Clegg said that he believes life in California will be "cool".

Sir Nick was first approached by Facebook in the Summer but rejected the offer. Sheryl Sandberg, the company's chief operating officer, subsequently called him while he was walking in the Alps during his Summer holidays to make a new approach.

He subsequently met Mr Zuckerberg and he and his wife then travelled to the West coast for dinner with the Facebook founder and Mr Zuckerberg.

Sir Nick told the BBC: "I said to them, if you're prepared to let me into the inner circle, in the black box, and me give real authority, then I'm interested."

He added: "The thing that persuaded me to do it is Mark and Sheryl were asking the right questions for the right reasons… about things like the barrier between free speech and prohibited content, wellbeing of children, integrity of elections, AI, and giving people control over their data.

"I thought, I could do this academically, as a commentator, running a start-up, but then I thought, in for a penny, in for a pound."